Methods for forming semiconductor device structures

ABSTRACT

The benefits of strained semiconductors are combined with silicon-on-insulator approaches to substrate and device fabrication.

RELATED APPLICATIONS

This application a continuation of U.S. Ser. No. 12/907,787, filed Oct.19, 2010, which is a divisional of U.S. Ser. No. 11/943,188, filed Nov.20, 2007, now U.S. Pat. No. 7,838,392, which is a continuationapplication of U.S. Ser. No. 11/127,508, filed May 12, 2005, now U.S.Pat. No. 7,297,612, which is a divisional application of U.S. Ser. No.10/456,103, filed Jun. 6, 2003, now U.S. Pat. No. 6,995,430, whichclaims the benefit of U.S. Provisional Application 60/386,968 filed Jun.7, 2002, U.S. Provisional Application 60/404,058 filed Aug. 15, 2002,and U.S. Provisional Application 60/416,000 filed Oct. 4, 2002; theentire disclosures of these nonprovisional utility patent applicationsand these provisional applications are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to devices and structures comprising strainedsemiconductor layers and insulator layers.

BACKGROUND

Strained silicon-on-insulator structures for semiconductor devicescombine the benefits of two advanced approaches to performanceenhancement: silicon-on-insulator (SOI) technology and strained silicon(Si) technology. The strained silicon-on-insulator configuration offersvarious advantages associated with the insulating substrate, such asreduced parasitic capacitances and improved isolation. Strained Siprovides improved carrier mobilities. Devices such as strained Simetal-oxide-semiconductor field-effect transistors (MOSFETs) combineenhanced carrier mobilities with the advantages of insulatingsubstrates.

Strained-silicon-on-insulator substrates are typically fabricated asfollows. First, a relaxed silicon-germanium (SiGe) layer is formed on aninsulator by one of several techniques such as separation byimplantation of oxygen (SIMOX), wafer bonding and etch back; waferbonding and hydrogen exfoliation layer transfer; or recrystallization ofamorphous material. Then, a strained Si layer is epitaxially grown toform a strained-silicon-on-insulator structure, with strained Sidisposed over SiGe. The relaxed-SiGe-on-insulator layer serves as thetemplate for inducing strain in the Si layer. This induced strain istypically greater than 10⁻³.

This structure has limitations. It is not conducive to the production offully-depleted strained-semiconductor-on-insulator devices in which thelayer over the insulating material must be thin enough [<300 angstroms(Å)] to allow for full depletion of the layer during device operation.Fully depleted transistors may be the favored version of SOI for MOSFETtechnologies beyond the 90 nm technology node. The relaxed SiGe layeradds to the total thickness of this layer and thus makes it difficult toachieve the thicknesses required for fully depleted silicon-on-insulatordevice fabrication. The relaxed SiGe layer is not required if a strainedSi layer can be produced directly on the insulating material. Thus,there is a need for a method to produce strained silicon—or othersemiconductor—layers directly on insulating substrates.

SUMMARY

The present invention includes a strained-semiconductor-on-insulator(SSOI) substrate structure and methods for fabricating the substratestructure. MOSFETs fabricated on this substrate will have the benefitsof SOI MOSFETs as well as the benefits of strained Si mobilityenhancement. By eliminating the SiGe relaxed layer traditionally foundbeneath the strained Si layer, the use of SSOI technology is simplified.For example, issues such as the diffusion of Ge into the strained Silayer during high temperature processes are avoided.

This approach enables the fabrication of well-controlled,epitaxially-defined, thin strained semiconductor layers directly on aninsulator layer. Tensile strain levels of ˜10⁻³ or greater are possiblein these structures, and are not diminished after thermal anneal cycles.In some embodiments, the strain-inducing relaxed layer is not present inthe final structure, eliminating some of the key problems inherent tocurrent strained Si-on-insulator solutions. This fabrication process issuitable for the production of enhanced- mobility substrates applicableto partially or fully depleted SSOI technology.

In an aspect, the invention features a structure including a substratehaving a dielectric layer disposed thereon; and a first strainedsemiconductor layer disposed in contact with the dielectric layer, thesemiconductor layer including approximately 100% germanium.

One or more of the following features may be included. The strainedsemiconductor layer may be compressively strained. The strainedsemiconductor layer may include a thin layer and the thin layer isdisposed in contact with the dielectric layer. The thin layer mayinclude silicon.

In another aspect, the invention features a substrate having adielectric layer disposed thereon, a strained semiconductor layerdisposed in contact with the dielectric layer, and a transistor. Thetransistor includes a source region and a drain region disposed in aportion of the strained semiconductor layer, and a gate disposed abovethe strained semiconductor layer and between the source and drainregions, the gate including a material selected from the groupconsisting of a doped semiconductor, a metal, and a metallic compound.

One or more of the following features may be included. The dopedsemiconductor may include polycrystalline silicon and/or polycrystallinesilicon-germanium. The metal may include titanium, tungsten, molybdenum,tantalum, nickel, and/or iridium. The metal compound may includetitanium nitride, titanium silicon nitride, tungsten nitride, tantalumnitride, tantalum silicide, nickel silicide, and/or iridium oxide. Acontact layer may be disposed over at least a portion of the strainedsemiconductor layer, with a bottommost boundary of the contact layerbeing disposed above a bottommost boundary of the strained semiconductorlayer. The contact layer may share an interface with the semiconductorlayer.

In another aspect, the invention features a structure including asubstrate having a dielectric layer disposed thereon, the dielectriclayer having a melting point greater than about 1700° C., and a strainedsemiconductor layer disposed in contact with the dielectric layer.

The following features may be included. The dielectric layer may includealuminum oxide, magnesium oxide, and/or silicon nitride.

In another aspect, the invention features a structure including asubstrate having a dielectric layer disposed thereon; and a strainedsemiconductor layer disposed in contact with the dielectric layer. Thestrained semiconductor layer includes approximately 100% silicon and hasa misfit dislocation density of less than about 10⁵ cm/cm². In anotheraspect, the invention features a structure including a substrate havinga dielectric layer disposed thereon, and a strained semiconductor layerdisposed in contact with the dielectric layer. The strainedsemiconductor layer includes approximately 100% silicon and has athreading dislocation density selected from the range of about 10dislocations/cm² to about 10⁷ dislocations/cm².

In another aspect, the invention features a structure including asubstrate having a dielectric layer disposed thereon and a strainedsemiconductor layer disposed in contact with the dielectric layer. Thesemiconductor layer includes approximately 100% silicon and has asurface roughness selected from the range of approximately 0.01 nm toapproximately 1 nm.

In another aspect, the invention features a substrate having adielectric layer disposed thereon, and a strained semiconductor layerdisposed in contact with the dielectric layer. The strainedsemiconductor layer includes approximately 100% silicon and has athickness uniformity across the substrate of better than approximately±10%.

In another aspect, the invention features a structure including asubstrate having a dielectric layer disposed thereon, and a strainedsemiconductor layer disposed in contact with the dielectric layer. Thestrained semiconductor layer includes approximately 100% silicon and hasa thickness of less than approximately 200 Å.

In another aspect, the invention features a structure including asubstrate having a dielectric layer disposed thereon, and a strainedsemiconductor layer disposed in contact with the dielectric layer. Thesemiconductor layer includes approximately 100% silicon and has asurface germanium concentration of less than approximately 1×10¹²atoms/cm².

In another aspect, the invention features a structure including asubstrate having a dielectric layer disposed thereon, and a strainedsemiconductor layer disposed in contact with the dielectric layer. Aninterface between the strained semiconductor layer and the dielectriclayer has a density of bonding voids of less than 0.3 voids/cm².

In another aspect, the invention features a method for forming astructure, the method including providing a first substrate comprising aporous layer defining a cleave plane and having a first strainedsemiconductor layer formed thereon. The first strained semiconductorlayer is bonded to an insulator layer disposed on a second substrate,and removing the first substrate from the first strained semiconductorlayer by cleaving at the cleave plane, the strained semiconductor layerremaining bonded to the insulator layer.

In another aspect, the invention features a method for forming astructure, the method including forming a first relaxed layer over afirst substrate, the first relaxed layer including a porous layerdefining a cleave plane. A strained semiconductor layer is formed overthe first relaxed layer. The first strained semiconductor layer isbonded to an insulator layer disposed on a second substrate. The firstsubstrate is removed from the strained semiconductor layer by cleavingat the cleave plane, the strained semiconductor layer remaining bondedto the insulator layer.

One or more of the following features may be included. The porous layermay be disposed at a top portion of the first relaxed layer. A secondrelaxed layer may be formed over the first relaxed layer, with thestrained semiconductor layer being formed over the second relaxed layer.The first relaxed layer may be planarized, e.g., by chemical-mechanicalpolishing, prior to forming the second relaxed layer. At least a portionof the porous layer may remain disposed on the first strainedsemiconductor layer after cleaving. The portion of the porous layer maybe removed from the strained semiconductor layer after cleaving. Theportion of the porous layer may be removed by cleaning with a wetchemical solution that may include, e.g., hydrogen peroxide and/orhydrofluoric acid. Removing the portion of the porous layer may includeoxidation.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A, 1B, 2A, 2B, and 3-6 are schematic cross-sectional views ofsubstrates illustrating a method for fabricating an SSOI substrate;

FIG. 7 is a schematic cross-sectional view illustrating an alternativemethod for fabricating the SSOI substrate illustrated in FIG. 6;

FIG. 8 is a schematic cross-sectional view of a transistor formed on theSSOI substrate illustrated in FIG. 6;

FIGS. 9-10 are schematic cross-sectional views of substrate(s)illustrating a method for fabricating an alternative SSOI substrate;

FIG. 11 is a schematic cross-sectional view of a substrate havingseveral layers formed thereon;

FIGS. 12-13 are schematic cross-sectional views of substratesillustrating a method for fabricating an alternative strainedsemiconductor substrate;

FIG. 14 is a schematic cross-sectional view of the SSOI substrateillustrated in FIG. 6 after additional processing; and

FIGS. 15A-16D are schematic cross-sectional views of substratesillustrating alternative methods for fabricating an SSOI substrate.

Like-referenced features represent common features in correspondingdrawings.

DETAILED DESCRIPTION

An SSOI structure may be formed by wafer bonding followed by cleaving.FIGS. 1A-2B illustrate formation of a suitable strained layer on a waferfor bonding, as further described below.

Referring to FIG. 1A, an epitaxial wafer 8 has a plurality of layers 10disposed over a substrate 12. Substrate 12 may be formed of asemiconductor, such as Si, Ge, or SiGe. The plurality of layers 10includes a graded buffer layer 14, which may be formed ofSi_(1-y)Ge_(y), with a maximum Ge content of, e.g., 10-80% (i.e.,y=0.1-0.8) and a thickness T₁ of, for example, 1-8 micrometers (μm).

A relaxed layer 16 is disposed over graded buffer layer 14. Relaxedlayer 16 may be formed of uniform Si_(1-x)Ge_(x) having a Ge content of,for example, 10-80% (i.e., x=0.1-0.8), and a thickness T₂ of, forexample, 0.2-2 μm. In some embodiments, Si_(1-x)Ge_(x) may includeSi_(0.70)Ge_(0.30) and T₂ may be approximately 1.5 μm. Relaxed layer 16may be fully relaxed, as determined by triple axis X-ray diffraction,and may have a threading dislocation density of <1×10⁶ dislocations/cm²,as determined by etch pit density (EPD) analysis. Because threadingdislocations are linear defects disposed within a volume of crystallinematerial, threading dislocation density may be measured as either thenumber of dislocations intersecting a unit area within a unit volume orthe line length of dislocation per unit volume. Threading dislocationdensity therefore, may, be expressed in either units of dislocations/cm²or cm/cm³. Relaxed layer 16 may have a surface particle density of,e.g., less than about 0.3 particles/cm². Further, relaxed layer 16produced in accordance with the present invention may have a localizedlight-scattering defect level of less than about 0.3 defects/cm² forparticle defects having a size (diameter) greater than 0.13 microns, adefect level of about 0.2 defects/cm² for particle defects having a sizegreater than 0.16 microns, a defect level of about 0.1 defects/cm² forparticle defects having a size greater than 0.2 microns, and a defectlevel of about 0.03 defects/cm² for defects having a size greater than 1micron. Process optimization may enable reduction of the localizedlight-scattering defect levels to about 0.09 defects/cm² for particledefects having a size greater than 0.09 microns and to 0.05 defects/cm²for particle defects having a size greater than 0.12 microns.

Substrate 12, graded layer 14, and relaxed layer 16 may be formed fromvarious materials systems, including various combinations of group II,group III, group IV, group V, and group VI elements. For example, eachof substrate 12, graded layer 14, and relaxed layer 16 may include aIII-V compound. Substrate 12 may include gallium arsenide (GaAs), gradedlayer 14 and relaxed layer 16 may include indium gallium arsenide(InGaAs) or aluminum gallium arsenide (AlGaAs). These examples aremerely illustrative, and many other material systems are suitable.

A strained semiconductor layer 18 is disposed over relaxed layer 16.Strained layer 18 may include a semiconductor such as at least one of agroup II, a group III, a group IV, a group V, and a group VI element.Strained semiconductor layer 18 may include, for example, Si, Ge, SiGe,GaAs, indium phosphide (InP), and/or zinc selenide (ZnSe). In someembodiments, strained semiconductor layer 18 may include approximately100% Ge, and may be compressively strained. Strained semiconductor layer18 comprising 100% Ge may be formed over, e.g., relaxed layer 16containing uniform Si_(1-x)Ge_(x) having a Ge content of, for example,50-80% (i.e., x=0.5-0.8), preferably 70% (x=0.7). Strained layer 18 hasa thickness T₃ of, for example, 50-1000 Å. In an embodiment, T₃ may beapproximately 200-500 Å.

Strained layer 18 may be formed by epitaxy, such as byatmospheric-pressure CVD (APCVD), low- (or reduced-) pressure CVD(LPCVD), ultra-high-vacuum CVD (UHVCVD), by molecular beam epitaxy(MBE), or by atomic layer deposition (ALD). Strained layer 18 containingSi may be formed by CVD with precursors such as silane, disilane, ortrisilane. Strained layer 18 containing Ge may be formed by CVD withprecursors such as germane or digermane. The epitaxial growth system maybe a single-wafer or multiple-wafer batch reactor. The growth system mayalso utilize a low-energy plasma to enhance layer growth kinetics.Strained layer 18 may be formed at a relatively low temperature, e.g.,less than 700° C., to facilitate the definition of an abrupt interface17 between strained layer 18 and relaxed layer 16. This abrupt interface17 may enhance the subsequent separation of strained layer 18 fromrelaxed layer 16, as discussed below with reference to FIGS. 4 and 5.Abrupt interface 17 is characterized by the transition of Si or Gecontent (in this example) proceeding in at least 1 decade (order ofmagnitude in atomic concentration) per nanometer of depth into thesample. In an embodiment, this abruptness may be better than 2 decadesper nanometer.

In an embodiment in which strained layer 18 contains substantially 100%Si, strained layer 18 may be formed in a dedicated chamber of adeposition tool that is not exposed to Ge source gases, thereby avoidingcross-contamination and improving the quality of the interface betweenstrained layer 18 and relaxed layer 16. Furthermore, strained layer 18may be formed from an isotopically pure silicon precursor(s).Isotopically pure Si has better thermal conductivity than conventionalSi. Higher thermal conductivity may help dissipate heat from devicessubsequently formed on strained layer 18, thereby maintaining theenhanced carrier mobilities provided by strained layer 18.

After formation, strained layer 18 has an initial misfit dislocationdensity, of, for example, 0-10⁵ cm/cm². In an embodiment, strained layer18 has an initial misfit dislocation density of approximately 0 cm/cm².Because misfit dislocations are linear defects generally lying within aplane between two crystals within an area, they may be measured in termsof total line length per unit area. Misfit dislocation density,therefore, may be expressed in units of dislocations/cm or cm/cm². Inone embodiment, strained layer 18 is tensilely strained, e.g., Si formedover SiGe. In another embodiment, strained layer 18 is compressivelystrained, e.g., Ge formed over SiGe.

Strained layer 18 may have a surface particle density of, e.g., lessthan about 0.3 particles/cm². As used herein, “surface particle density”includes not only surface particles but also light-scattering defects,and crystal-originated pits (COPs), and other defects incorporated intostrained layer 18. Further, strained layer 18 produced in accordancewith the present invention may have a localized light-scattering defectlevel of less than about 0.3 defects/cm² for particle defects having asize (diameter) greater than 0.13 microns, a defect level of about 0.2defects/cm² for particle defects having a size greater than 0.16microns, a defect level of about 0.1 defects/cm² for particle defectshaving a size greater than 0.2 microns, and a defect level of about 0.03defects/cm² for defects having a size greater than 1 micron. Processoptimization may enable reduction of the localized light-scatteringdefect levels to about 0.09 defects/cm² for particle defects having asize greater than 0.09 microns and to 0.05 defects/cm² for particledefects having a size greater than 0.12 microns. These surface particlesmay be incorporated in strained layer 18 during the formation ofstrained layer 18, or they may result from the propagation of surfacedefects from an underlying layer, such as relaxed layer 16.

In alternative embodiments, graded layer 14 may be absent from thestructure. Relaxed layer 16 may be formed in various ways, and theinvention is not limited to embodiments having graded layer 14. In otherembodiments, strained layer 18 may be formed directly on substrate 12.In this case, the strain in layer 18 may be induced by lattice mismatchbetween layer 18 and substrate 12, induced mechanically, e.g., by thedeposition of overlayers, such as Si₃N₄, or induced by thermal mismatchbetween layer 18 and a subsequently grown layer, such as a SiGe layer.In some embodiments, a uniform semiconductor layer (not shown), having athickness of approximately 0.5 μm and comprising the same semiconductormaterial as substrate 12, is disposed between graded buffer layer 14 andsubstrate 12. This uniform semiconductor layer may be grown to improvethe material quality of layers subsequently grown on substrate 12, suchas graded buffer layer 14, by providing a clean, contaminant-freesurface for epitaxial growth. In certain embodiments, relaxed layer 16may be planarized prior to growth of strained layer 18 to eliminate thecrosshatched surface roughness induced by graded buffer layer 14. (See,e.g., M. T. Currie, et al., Appl. Phys. Lett., 72 (14) p. 1718 (1998),incorporated herein by reference.) The planarization may be performed bya method such as chemical mechanical polishing (CMP), and may improvethe quality of a subsequent bonding process (see below) because itminimizes the wafer surface roughness and increases wafer flatness, thusproviding a greater surface area for bonding.

Referring to FIG. 1B, after planarization of relaxed layer 16, a relaxedsemiconductor regrowth layer 19 including a semiconductor such as SiGemay be grown on relaxed layer 16, thus improving the quality ofsubsequent strained layer 18 growth by ensuring a clean surface for thegrowth of strained layer 18. Growing on this clean surface may bepreferable to growing strained material, e.g., silicon, on a surfacethat is possibly contaminated by oxygen and carbon from theplanarization process. The conditions for epitaxy of the relaxedsemiconductor regrowth layer 19 on the planarized relaxed layer 16should be chosen such that surface roughness of the resulting structure,including layers formed over regrowth layer 19, is minimized to ensure asurface suitable for subsequent high quality bonding. High qualitybonding may be defined as the existence of a bond between two wafersthat is substantially free of bubbles or voids at the interface.Measures that may help ensure a smooth surface for strained layer 18growth, thereby facilitating bonding, include substantially matching alattice of the semiconductor regrowth layer 19 to that of the underlyingrelaxed layer 16, by keeping the regrowth thickness below approximately1 μm, and/or by keeping the growth temperature below approximately 850°C. for at least a portion of the semiconductor layer 19 growth. It mayalso be advantageous for relaxed layer 16 to be substantially free ofparticles or areas with high threading dislocation densities (i.e.,threading dislocation pile-ups) which could induce non-planarity in theregrowth and decrease the quality of the subsequent bond.

Referring to FIG. 2A, in an embodiment, hydrogen ions are implanted intorelaxed layer 16 to define a cleave plane 20. This implantation issimilar to the SMARTCUT process that has been demonstrated in siliconby, e.g., SOITEC, based in Grenoble, France. Implantation parameters mayinclude implantation of hydrogen (H₂ ⁺) to a dose of 2.5-5×10¹⁶ ions/cm²at an energy of, e.g., 50-100 keV. For example, H₂ ⁺ may be implanted atan energy of 75 keV and a dose of 4×10¹⁶ ions/cm² through strained layer18 into relaxed layer 16. In alternative embodiments, it may befavorable to implant at energies less than 50 keV to decrease the depthof cleave plane 20 and decrease the amount of material subsequentlyremoved during the cleaving process (see discussion below with referenceto FIG. 4). In an alternative embodiment, other implanted species may beused, such as H⁺ or He⁺, with the dose and energy being adjustedaccordingly. The implantation may also be performed prior to theformation of strained layer 18. Then, the subsequent growth of strainedlayer 18 is preferably performed at a temperature low enough to preventpremature cleaving along cleave plane 20, i.e., prior to the waferbonding process. This cleaving temperature is a complex function of theimplanted species, implanted dose, and implanted material. Typically,premature cleaving may be avoided by maintaining a growth temperaturebelow approximately 500° C.

In some embodiments, such as when strained layer 18 comprises nearly100% Ge, a thin layer 21 of another material, such as Si, may be formedover strained layer 18 prior to bonding (see discussion with respect toFIG. 3). This thin layer 21 may be formed to enhance subsequent bondingof strained layer 18 to an insulator, such as an oxide. Thin layer 21may have a thickness T₂₁ of, for example, 0.5-5 nm.

In some embodiments, strained layer 18 may be planarized by, e.g., CMP,to improve the quality of the subsequent bond. Strained layer 18 mayhave a low surface roughness, e.g., less than 0.5 nm root mean square(RMS). Referring to FIG. 2B, in some embodiments, a dielectric layer 22may be formed over strained layer 18 prior to ion implantation intorelaxed layer 16 to improve the quality of the subsequent bond.Dielectric layer 22 may be, e.g., silicon dioxide (SiO₂) deposited by,for example, LPCVD or by high density plasma (HDP). An LPCVD depositedSiO₂ layer may be subjected to a densification step at elevatedtemperature. Suitable conditions for this densification step may be, forexample, a 10 minute anneal at 800° C. in a nitrogen ambient.Alternatively, dielectric layer 22 may include low-temperature oxide(LTO), which may be subsequently densified at elevated temperature innitrogen or oxygen ambients. Suitable conditions for this densificationstep can be a 10 minute anneal at 800° C. in an oxygen ambient.Dielectric layer 22 may be planarized by, e.g., CMP to improve thequality of the subsequent bond. In an alternative embodiment, it may beadvantageous for dielectric layer 22 to be formed from thermally grownSiO₂ in order to provide a high quality semiconductor/dielectricinterface in the final structure. In an embodiment, strained layer 18comprises approximately 100% Ge and dielectric layer 22 comprises, forexample, germanium dioxide (GeO₂); germanium oxynitride (GeON); a high-kinsulator having a higher dielectric constant than that of SiO₂ such ashafnium oxide (HfO₂) or hafnium silicate (HfSiON, HfSiO₄); or amultilayer structure including GeO₂ and SiO₂. Ge has an oxidationbehavior different from that of Si, and the deposition methods may bealtered accordingly.

Referring to FIG. 3, epitaxial wafer 8 is bonded to a handle wafer 50.Either handle wafer 50, epitaxial wafer 8, or both have a top dielectriclayer (see, e.g., dielectric layer 22 in FIG. 2B) to facilitate thebonding process and to serve as an insulator layer in the finalsubstrate structure. Handle wafer 50 may have a dielectric layer 52disposed over a semiconductor substrate 54. Dielectric layer 52 mayinclude, for example, SiO₂. In an embodiment, dielectric layer 52includes a material having a melting point (T_(m)) higher than a T_(m)of pure SiO₂, i.e., higher than 1700° C. Examples of such materials aresilicon nitride (Si₃N₄), aluminum oxide, magnesium oxide, etc. Usingdielectric layer 52 with a high T_(m) helps prevent possible relaxationof the transferred strained semiconductor layer 18 that may occur duringsubsequent processing, due to softening of the underlying dielectriclayer 52 at temperatures typically used during device fabrication(approximately 1000-1200° C.). In other embodiments, handle wafer 50 mayinclude a combination of a bulk semiconductor material and a dielectriclayer, such as a silicon on insulator substrate. Semiconductor substrate54 includes a semiconductor material such as, for example, Si, Ge, orSiGe.

Handle wafer 50 and epitaxial wafer 8 are cleaned by a wet chemicalcleaning procedure to facilitate bonding, such as by a hydrophilicsurface preparation process to assist the bonding of a semiconductormaterial, e.g., strained layer 18, to a dielectric material, e.g.,dielectric layer 52. For example, a suitable prebonding surfacepreparation cleaning procedure could include a modified megasonic RCASC1 clean containing ammonium hydroxide, hydrogen peroxide, and water(NH₄OH:H₂O₂:H₂O) at a ratio of 1:4:20 at 60° C. for 10 minutes, followedby a deionized (DI) water rinse and spin dry. The wafer bonding energyshould be strong enough to sustain the subsequent layer transfer (seeFIG. 4). In some embodiments, top surfaces 60, 62 of handle wafer 50 andepitaxial wafer 8, including a top surface 63 of strained semiconductorlayer 18, may be subjected to a plasma activation, either before, after,or instead of a wet clean, to increase the bond strength. The plasmaenvironment may include at least one of the following species: oxygen,ammonia, argon, nitrogen, diborane, and phosphine. After an appropriatecleaning step, handle wafer 50 and epitaxial wafer 8 are bonded togetherby bringing top surfaces 60, 62 in contact with each other at roomtemperature. The bond strength may be greater than 1000 mJ/m², achievedat a low temperature, such as less than 600° C.

Referring to FIG. 4 as well as to FIG. 3, a split is induced at cleaveplane 20 by annealing handle wafer 50 and epitaxial wafer 8 after theyare bonded together. This split may be induced by an anneal at 300-700°C., e.g., 550° C., inducing hydrogen exfoliation layer transfer (i.e.,along cleave plane 20) and resulting in the formation of two separatewafers 70, 72. One of these wafers (70) has a first portion 80 ofrelaxed layer 16 (see FIG. 1A) disposed over strained layer 18. Strainedlayer 18 is in contact with dielectric layer 52 on semiconductorsubstrate 54. The other of these wafers (72) includes substrate 12,graded layer 14, and a remaining portion 82 of relaxed layer 16. In someembodiments, wafer splitting may be induced by mechanical force inaddition to or instead of annealing. If necessary, wafer 70 withstrained layer 18 may be annealed further at 600-900° C., e.g., at atemperature greater than 800° C., to strengthen the bond between thestrained layer 18 and dielectric layer 52. In some embodiments, thisanneal is limited to an upper temperature of about 900° C. to avoid thedestruction of a strained Si/relaxed SiGe heterojunction by diffusion.Wafer 72 may be planarized, and used as starting substrate 8 for growthof another strained layer 18. In this manner, wafer 72 may be “recycled”and the process illustrated in FIGS. 1A-5 may be repeated. Analternative “recyling” method may include providing relaxed layer 16that is several microns thick and repeating the process illustrated inFIGS. 1A-5, starting with the formation of strained layer 18. Becausethe formation of this thick relaxed layer 16 may lead to bowing ofsubstrate 12, a layer including, e.g., oxide or nitride, may be formedon the backside of substrate 12 to counteract the bowing. Alternativelysubstrate 12 may be pre-bowed when cut and polished, in anticipation ofthe bow being removed by the formation of thick relaxed layer 16.

Referring to FIG. 4 as well as to FIG. 5, relaxed layer portion 80 isremoved from strained layer 18. In an embodiment, removal of relaxedlayer portion 80, containing, e.g., SiGe, includes oxidizing the relaxedlayer portion 80 by wet (steam) oxidation. For example, at temperaturesbelow approximately 800° C., such as temperatures between 600-750° C.,wet oxidation will oxidize SiGe much more rapidly then Si, such that theoxidation front will effectively stop when it reaches the strained layer18, in embodiments in which strained layer 18 includes Si. Thedifference between wet oxidation rates of SiGe and Si may be evengreater at lower temperatures, such as approximately 400° C.-600° C.Good oxidation selectivity is provided by this difference in oxidationrates, i.e., SiGe may be efficiently removed at low temperatures withoxidation stopping when strained layer 18 is reached. This wet oxidationresults in the transformation of SiGe to a thermal insulator 90, e.g.,Si_(x)Ge_(y)O_(z). The thermal insulator 90 resulting from thisoxidation is removed in a selective wet or dry etch, e.g., wethydrofluoric acid. In some embodiments, it may be more economical tooxidize and strip several times, instead of just once.

In certain embodiments, wet oxidation may not completely remove therelaxed layer portion 80. Here, a localized rejection of Ge may occurduring oxidation, resulting in the presence of a residual Ge-rich SiGeregion at the oxidation front, on the order of, for example, severalnanometers in lateral extent. A surface clean may be performed to removethis residual Ge. For example, the residual Ge may be removed by a dryoxidation at, e.g., 600° C., after the wet oxidation and strip describedabove. Another wet clean may be performed in conjunction with—or insteadof—the dry oxidation. Examples of possible wet etches for removingresidual Ge include a Piranha etch, i.e., a wet etch that is a mixtureof sulfuric acid and hydrogen peroxide (H₂SO₄:H₂O₂) at a ratio of, forexample, 3:1. An HF dip may be performed after the Piranha etch.Alternatively, an RCA SC1 clean may be used to remove the residual Ge.The process of Piranha or RCA SC1 etching and HF removal of resultingoxide may be repeated more than once. In an embodiment, relaxed layerportion including, e.g., SiGe, is removed by etching and annealing undera hydrochloric acid (HCl) ambient.

In the case of a strained Si layer, the surface Ge concentration of thefinal strained Si surface is preferably less than about 1×10² atoms/cm²when measured by a technique such as total reflection x-ray fluorescence(TXRF) or the combination of vapor phase decomposition (VPD) with aspectroscopy technique such as graphite furnace atomic absorptionspectroscopy (GFAAS) or inductively-coupled plasma mass spectroscopy(ICP-MS). In some embodiments, after cleaving, a planarization step or awet oxidation step may be performed to remove a portion of the damagedrelaxed layer portion 80 as well as to increase the smoothness of itssurface. A smoother surface may improve the uniformity of subsequentcomplete removal of a remainder of relaxed layer portion 80 by, e.g.,wet chemical etching. After removal of relaxed layer portion 80,strained layer 18 may be planarized. Planarization of strained layer 18may be performed by, e.g., CMP; an anneal at a temperature greater than,for example, 800° C., in a hydrogen (H₂) or hydrochloric acid (HCl)containing ambient; or cluster ion beam smoothing.

Referring to FIG. 6, a SSOI substrate 100 has strained layer 18 disposedover an insulator, such as dielectric layer 52 formed on semiconductorsubstrate 54. Strained layer 18 has a thickness T₄ selected from a rangeof, for example, 50-1000 Å, with a thickness uniformity of better thanapproximately ±5% and a surface roughness of less than approximately 20Å. Dielectric layer 52 has a thickness T₅₂ selected from a range of, forexample, 500-3000 Å. In an embodiment, strained layer 18 includesapproximately 100% Si or 100% Ge having one or more of the followingmaterial characteristics: misfit dislocation density of, e.g., 0-10⁵cm/cm²; a threading dislocation density of about 10¹-10⁷dislocations/cm²; a surface roughness of approximately 0.01-1 nm RMS;and a thickness uniformity across SSOI substrate 100 of better thanapproximately ±10% of a mean desired thickness; and a thickness T₄ ofless than approximately 200 Å. In an embodiment, SSOI substrate 100 hasa thickness uniformity of better than approximately ±5% of a meandesired thickness.

In an embodiment, dielectric layer 52 has a T_(m) greater than that ofSiO₂. During subsequent processing, e.g., MOSFET formation, SSOIsubstrate 100 may be subjected to high temperatures, i.e., up to 1100°C. High temperatures may result in the relaxation of strained layer 18at an interface between strained layer 18 and dielectric layer 52. Theuse of dielectric layer with a T_(m) greater than 1700° C. may help keepstrained layer 18 from relaxing at the interface between strained layer18 and dielectric layer 52 when SSOI substrate is subjected to hightemperatures.

In an embodiment, the misfit dislocation density of strained layer 18may be lower than its initial dislocation density. The initialdislocation density may be lowered by, for example, performing an etchof a top surface 92 of strained layer 18. This etch may be a wet etch,such as a standard microelectronics clean step such as an RCA SC1, i.e.,hydrogen peroxide, ammonium hydroxide, and water (H₂O₂+NH₄OH+H₂O), whichat, e.g., 80° C. may remove silicon.

The presence of surface particles on strained layer 18, as describedabove with reference to FIG. 1A, may result in the formation of bondingvoids at an interface 102 between strained layer 18 and dielectric layer52. These bonding voids may have a density equivalent to the density ofsurface particles formed on strained layer 18, e.g., less than about 0.3voids/cm².

In some embodiments, strained semiconductor layer 18 includes Si and issubstantially free of Ge; further, any other layer disposed in contactwith strained semiconductor layer 18 prior to device processing, e.g.,dielectric layer 52, is also substantially free of Ge.

Referring to FIG. 7, in an alternative embodiment, relaxed layer portion80 may be removed by a selective wet etch that stops at the strainedlayer 18 to obtain SSOI substrate 100 (see FIG. 6). In embodiments inwhich relaxed layer portion 80 contains SiGe, a suitable selective SiGewet etch may be a solution containing nitric acid (HNO₃) and dilute HFat a ratio of 3:1 or a solution containing H₂O₂, HF, and acetic acid(CH₃COOH) at a ratio of 2:1:3. Alternatively, relaxed layer portion 80may be removed by a dry etch that stops at strained layer 18. In someembodiments, relaxed layer portion 80 may be removed completely or inpart by a chemical-mechanical polishing step or by mechanical grinding.

Strained semiconductor-on-insulator substrate 100 may be furtherprocessed by CMOS SOI MOSFET fabrication methods. For example, referringto FIG. 8A, a transistor 200 may be formed on SSOI substrate 100.Forming transistor 200 includes forming a gate dielectric layer 210above strained layer 18 by, for example, growing an SiO₂ layer bythermal oxidation. Alternatively, gate dielectric layer 210 may includea high-k material with a dielectric constant higher than that of SiO₂,such as HfO₂, HfSiON, or HfSiO₄. In some embodiments, gate dielectriclayer 210 may be a stacked structure, e.g., a thin SiO₂ layer cappedwith a high-k material. A gate 212 is formed over gate dielectric layer210. Gate 212 may be formed of a conductive material, such as dopedsemiconductor, e.g., polycrystalline Si or polycrystalline SiGe; ametal, e.g., titanium (Ti), tungsten (W), molybdenum (Mo), tantalum(Ta), nickel (Ni), or iridium (Ir); or metal compounds, e.g., titaniumnitride (TiN), titanium silicon nitride (TiSiN), tungsten nitride (WN),tantalum nitride (TaN), tantalum silicide (TaSi), nickel silicide(NiSi), or iridium oxide (IrO₂), that provide an appropriateworkfunction. A source region 214 and a drain region 216 are formed in aportion 218 of strained semiconductor layer 18, proximate gatedielectric layer 210. Source and drain regions 214, 216 may be formedby, e.g., ion implantation of either n-type or p-type dopants.

In some embodiments, strained semiconductor layer 18 may becompressively strained when, for example, layer 18 includes strained Ge.Compressively strained layers may be prone to undulation when subjectedto large temperature changes. The risk of such undulation may be reducedby reducing the thermal budget of a process for fabricating devices,such as transistor 200. The thermal budget may be reduced by, forexample, using atomic layer deposition (ALD) to deposit gate dielectriclayer 210. Furthermore, a maximum temperature for forming gate 212 maybe limited to, e.g., 600° C. by, for example, the use of materialscomprising metal or metal compounds, rather than polysilicon or othergate materials that may require higher formation and/or dopantactivation temperatures.

Referring to FIG. 8B, a transistor 250 formed on SSOI substrate 100 mayhave an elevated source region and an elevated drain region proximate afirst and a second sidewall spacer 252, 254. These elevated regions maybe formed as follows. A semiconductor layer 256 a-256 c is formedselectively on exposed silicon surfaces, i.e., on top surface 258 of agate 259 containing silicon, a top surface 260 of a source 262 definedin strained layer 18, and top surface 264 of a drain 266 defined instrained layer 18. In an embodiment, semiconductor layer 256 a-256 c isan epitaxial layer, such as epitaxial silicon, epitaxial germanium, orepitaxial silicon-germanium. No semiconductor layer is formed onnon-silicon features, such as sidewall spacers 252, 254 and dielectricisolation regions 268, 270. Semiconductor layer 256 a-256 c has athickness T₂₅₆ of, for example, approximately 100-500 Å.

Semiconductor layer 256 a-256 c has a low resistivity of, e.g., 0.001ohm-cm, that facilitates the formation of low-resistance contacts. Toachieve this low resistivity, semiconductor layer 256 a-256 c is, forexample, epitaxial silicon doped with, for example, arsenic to aconcentration of 1×10²⁰ atoms/cm³. Semiconductor layer 256 a-256 c maybe doped in situ, during deposition. In alternative embodiments,semiconductor layer 256 a-256 c may be doped after deposition by ionimplantation or by gas-, plasma- or solid-source diffusion. In someembodiments, the doping of semiconductor layer 256 a-256 c and theformation of source 262 and drain 266 are performed simultaneously.Portions of semiconductor layer 256 a, 256 c disposed over source 262and drain 266 may have top surfaces substantially free of facets. In anembodiment, portions of source 262, drain 266, and/or gate 259 may beetched away to define recess prior to deposition of semiconductor layer256 a-256 c, and semiconductor layer 256 a-256 c may then be depositedin the recesses thus formed.

Referring to FIG. 8C, a metal layer 272 is formed over transistor 250.Metal layer 272 is formed by, for example, sputter deposition. Metallayer 272 has a thickness T₂₇₂ of, e.g., 50-200 Å and includes a metalsuch as cobalt, titanium, tungsten, nickel, or platinum. The metal isselected to react with semiconductor layer 256 a-256 c to form alow-resistance metal-semiconductor alloy when exposed to heat, asdescribed below. The metal is also selected such that themetal-semiconductor alloy remains stable at temperatures typicallyrequired to complete transistor 250 fabrication, e.g., 400-700° C.

Referring also to FIG. 8D, subsequent to deposition of metal layer 272,a first rapid thermal anneal is performed, e.g., at 550° C. for 60seconds. This heating step initiates a reaction between metal layer 272and semiconductor layers 256 a-256 c, forming a high resistivity phaseof a metal-semiconductor alloy, e.g., cobalt silicide (CoSi). Portionsof metal layer 272 are removed by a wet etch, such as sulfuric acid andhydrogen peroxide. In an alternative embodiment, the wet etch may beammonium hydroxide, peroxide, and water. This wet etch removes portionsof metal layer 272 disposed over dielectric material, such as over firstand second sidewall spacers 252, 254 and isolation regions 268, 270.Portions 274 of metal layer 272 disposed over semiconductor layer 256a-256 c that have reacted to form the metal-semiconductor alloy remainin place after the anneal and wet etch.

Referring to FIG. 8E, SSOI substrate 100, including transistor 250, issubjected to a second heat treatment. For example, in an embodiment inwhich metal layer 272 includes cobalt, SSOI substrate 100 undergoes arapid thermal anneal at 800° C. for 60 seconds in a nitrogen ambient.This heating step initiates a reaction in the metal-semiconductor alloylayer which substantially lowers its resistivity, to form asubstantially homogeneous contact layer 276 a-276 c. Contact layer 276a-276 c includes a metal-semiconductor alloy, e.g., a metal silicidesuch as a low resistivity phase of cobalt silicide (CoSi₂). Contactlayer 276 a-276 c has a thickness T₂₇₆ of, for example, 400 Å. Contactlayer 276 a-276 c has a low sheet resistance, e.g., less than about 10Ω/, and enables a good quality contact to be made to source 262 anddrain 266, as well as to gate 259.

In some embodiments, during formation, contact layer 276 a-276 c mayconsume substantially all of semiconductor layer 256 a-256 c. Abottommost boundary 278 a of contact layer 276 a, therefore, shares aninterface 280 a with strained layer 18 in source 262, and a bottommostboundary 278 c of contact layer 276 c, therefore, shares an interface280 c with strained layer 18 in drain 266. A bottommost boundary 278 bof contact layer 276 b shares an interface 280 b with gate 259.

In other embodiments, contact layer portions 276 a, 276 c, disposed oversource 262 and drain 266, may extend into strained layer 18. Interfaces280 a, 280 c between contact layer 276 a, 276 c and strained layer 18are then disposed within source 262 and drain 266, respectively, abovebottommost boundaries 282 a, 282 c of strained layer 18. Interfaces 280a, 280 c have a low contact resistivity, e.g., less than approximately5×10⁷ Ω-cm². In certain other embodiments, during formation, contactlayer 276 a-276 c may not consume all of semiconductor layer 256 a-256 c(see FIG. 8D). A bottommost boundary 278 a of contact layer 276 a,therefore, shares an interface with semiconductor layer 256 a oversource 262, and a bottommost boundary 278 c of contact layer 276 c,therefore, shares an interface with semiconductor layer 256 c over drain266.

Because strained layer 18 includes a strained material, carriermobilities in strained layer 18 are enhanced, facilitating lower sheetresistances. This strain also results in a reduced energy bandgap,thereby lowering the contact resistivity between the metal-semiconductoralloy and the strained layer.

In alternative embodiments, an SSOI structure may include, instead of asingle strained layer, a plurality of semiconductor layers disposed onan insulator layer. For example, referring to FIG. 9, epitaxial wafer300 includes strained layer 18, relaxed layer 16, graded layer 14, andsubstrate 12. In addition, a semiconductor layer 310 is disposed overstrained layer 18. Strained layer 18 may be tensilely strained andsemiconductor layer 310 may be compressively strained. In an alternativeembodiment, strained layer 18 may be compressively strained andsemiconductor layer 310 may be tensilely strained. Strain may be inducedby lattice mismatch with respect to an adjacent layer, as describedabove, or mechanically. For example, strain may be induced by thedeposition of overlayers, such as Si₃N₄. In another embodiment,semiconductor layer 310 is relaxed. Semiconductor layer 310 includes asemiconductor material, such as at least one of a group II, a group III,a group IV, a group V, and a group VI element. Epitaxial wafer 300 isprocessed in a manner analogous to the processing of epitaxial wafer 8,as described with reference to FIGS. 1-7.

Referring also to FIG. 10, processing of epitaxial wafer 300 results inthe formation of SSOI substrate 350, having strained layer 18 disposedover semiconductor layer 310. Semiconductor layer 310 is bonded todielectric layer 52, disposed over substrate 54. As noted above withreference to FIG. 9, strained layer 18 may be tensilely strained andsemiconductor layer 310 may be compressively strained. Alternatively,strained layer 18 may be compressively strained and semiconductor layer310 may be tensilely strained. In some embodiments, semiconductor layer310 may be relaxed.

Referring to FIG. 11, in some embodiments, a thin strained layer 84 maybe grown between strained layer 18 and relaxed layer 16 to act as anetch stop during etching, such as wet etching. In an embodiment in whichstrained layer 18 includes Si and relaxed layer 16 includesSi_(1-y)Ge_(y), thin strained layer 84 may include Si_(1-x)Ge_(x), witha higher Ge content (x) than the Ge content (y) of relaxed layer 16, andhence be compressively strained. For example, if the composition of therelaxed layer 16 is 20% Ge (Si_(0.80)Ge_(0.20)), thin strained layer 84may contain 40% Ge (Si_(0.60)Ge_(0.40)) to provide a more robust etchstop. In other embodiments, a second strained layer, such as thinstrained layer 84 with higher Ge content than relaxed layer 16, may actas a preferential cleave plane in the hydrogen exfoliation/cleavingprocedure described above.

In an alternative embodiment, thin strained layer 84 may containSi_(1-x)Ge_(x) with lower Ge content than relaxed layer 16. In thisembodiment, thin strained layer 84 may act as a diffusion barrier duringthe wet oxidation process. For example, if the composition of relaxedlayer 16 is 20% Ge (Si_(0.80)Ge_(0.20)), thin strained layer 84 maycontain 10% Ge (Si_(0.90)Ge_(0.10)) to provide a barrier to Ge diffusionfrom the higher Ge content relaxed layer 16 during the oxidationprocess. In another embodiment, thin strained layer 84 may be replacedwith a thin graded Si_(1-z)Ge_(z) layer in which the Ge composition (z)of the graded layer is decreased from relaxed layer 16 to the strainedlayer 18.

Referring again to FIG. 7, in some embodiments, a small amount, e.g.,approximately 20-100 Å, of strained layer 18 may be removed at aninterface 105 between strained layer 18 and relaxed layer portion 80.This may be achieved by overetching after relaxed layer portion 80 isremoved. Alternatively, this removal of strained layer 18 may beperformed by a standard microelectronics clean step such as an RCA SC1,i.e., hydrogen peroxide, ammonium hydroxide, and water (H₂O₂+NH₄OH+H₂O),which at, e.g., 80° C. may remove silicon. This silicon removal mayremove any misfit dislocations that formed at the original strainedlayer 18/relaxed layer 80 interface 105 if strained layer 18 was grownabove the critical thickness. The critical thickness may be defined asthe thickness of strained layer 18 beyond which it becomes energeticallyfavorable for the strain in the layer to partially relax via theintroduction of misfit dislocations at interface 105 between strainedlayer 18 and relaxed layer 16. Thus, the method illustrated in FIGS. 1-7provides a technique for obtaining strained layers above a criticalthickness without misfit dislocations that may compromise theperformance of deeply scaled MOSFET devices.

Referring to FIG. 12, in some embodiments, handle wafer 50 may have astructure other than a dielectric layer 52 disposed over a semiconductorsubstrate 54. For example, a bulk relaxed substrate 400 may comprise abulk material 410 such as a semiconductor material, e.g., bulk silicon.Alternatively, bulk material 410 may be a bulk dielectric material, suchas Al₂O₃ (e.g., alumina or sapphire) or SiO₂ (e.g., quartz). Epitaxialwafer 8 may then be bonded to handle wafer 400 (as described above withreference to FIGS. 1-6), with strained layer 18 being bonded to the bulkmaterial 410 comprising handle wafer 400. In embodiments in which bulkmaterial 410 is a semiconductor, to facilitate thissemiconductor-semiconductor bond, a hydrophobic clean may be performed,such as an HF dip after an RCA SC1 clean.

Referring to FIG. 13, after bonding and further processing (as describedabove), a strained-semiconductor-on-semiconductor (SSOS) substrate 420is formed, having strained layer 18 disposed in contact with relaxedsubstrate 400. The strain of strained layer 18 is not induced byunderlying relaxed substrate 400, and is independent of any latticemismatch between strained layer 18 and relaxed substrate 400. In anembodiment, strained layer 18 and relaxed substrate 400 include the samesemiconductor material, e.g., silicon. Relaxed substrate 400 may have alattice constant equal to a lattice constant of strained layer 18 in theabsence of strain. Strained layer 18 may have a strain greater thanapproximately 1×10⁻³. Strained layer 18 may have been formed by epitaxy,and may have a thickness T₅ of between approximately 20 Å-1000 Å, with athickness uniformity of better than approximately ±10%. In anembodiment, strained layer 18 may have a thickness uniformity of betterthan approximately ±5%. Surface 92 of strained layer 18 may have asurface roughness of less than 20 Å.

Referring to FIG. 14, in an embodiment, after fabrication of the SSOIstructure 100 including semiconductor substrate 54 and dielectric layer52, it may be favorable to selectively relax the strain in at least aportion of strained layer 18. This could be accomplished by introducinga plurality of ions by, e.g., ion implantation after a photolithographystep in which at least a portion of the structure is masked by, forexample, a photoresist feature 500. Ion implantation parameters may be,for example, an implant of Si ions at a dose of 1×10¹⁵-1×10¹⁷ ions/cm²,at an energy of 5-75 keV. After ion implantation, a relaxed portion 502of strained layer 18 is relaxed, while a strained portion 504 ofstrained layer 18 remains strained.

Referring to FIGS. 15A-15E, SSOI structure 100 (see FIG. 6) may beformed by the use of a porous semiconductor substrate. Referring to FIG.15A, substrate 12 may be formed of a semiconductor, such as Si, Ge, orSiGe. A plurality of pores 1514, i.e., microvoids, are formed to definea porous layer 1516 in a portion of substrate 12. Pores 1514 may have amedian diameter of 5-10 nm and a pitch of 10-50 nm. Porous layer 1516may have a porosity of 10-50% and may extend a depth of d₁₅ intosubstrate 12 of approximately 1-5 μm.

Referring to FIG. 15B, pores 1514 may be formed by, for example,submerging substrate 12 into a vessel 1517 containing an electrolyte1518, such as hydrofluoric acid (HF), possibly mixed with ethanol, witha cathode 1520 and an anode 1522 disposed in the electrolyte 1518. Aback surface chucking holder 1519 a with a vacuum pad 1519 b may holdsubstrate 12 while it is submerged in vessel 1517. A current may begenerated between cathode 1520 and anode 1522, through substrate 12,resulting in the electrochemical etching of substrate 12, therebyforming pores 1514 at a top surface 1524 of substrate 12. In anembodiment, prior to the formation of pores 1514, substrate 12 may beplanarized, e.g., by CMP.

Referring to FIG. 15C, after the formation of pores 1514, a plurality oflayers 10 may be formed over porous top surface 1524 of substrate 12, asdescribed with reference to FIG. 1A. Layers 10 may include, for example,graded buffer layer 14, relaxed layer 16, and strained layer 18. Pores1514 define cleave plane 20 in porous layer 1516 of substrate 12.

Referring to FIG. 15D, substrate 12 with layers 10 is bonded to handlewafer 50, including semiconductor substrate 54 and dielectric layer 52,as described with reference to FIG. 3. Prior to bonding, a dielectriclayer may be formed on a top surface of layers 10 to facilitate thebonding process and to serve as an insulator layer in the finalsubstrate structure.

Referring to FIG. 15E as well as to FIG. 15D, a split is induced atcleave plane 20 by, for example, cleaving porous layer 1516 by a wateror an air jet. The split results in the formation of two separate wafers1570, 1572. One of these wafers (1572) has graded layer 14 and relaxedlayer 16 (see FIG. 15C) disposed over strained layer 18, with a firstportion 1580 of substrate 12 disposed over graded layer 14. Firstportion 1580 of substrate 12 may be just trace amounts of materialsurrounding pores 1514. Strained layer 18 is in contact with dielectriclayer 52 on semiconductor substrate 54. The other of these wafers (1570)includes a second portion 1582 of substrate 12, including the bulk ofsubstrate 12 with perhaps trace amounts of material surrounding pores1514.

Referring to FIG. 6 as well as to FIG. 15E, first portion 1580 ofsubstrate 12 is removed from graded layer 14 by a wet chemical cleaningprocess utilizing, for example a mixture of hydrogen peroxide (H₂O₂) andHF. Graded layer 14 and relaxed layer 16 are removed in any one of themethods described for the removal of relaxed layer portion 80 withreference to FIGS. 4 and 5. Removal of graded and relaxed layers 14, 16results in the formation of SSOI substrate 100.

Referring to FIG. 16A, SSOI substrate 100 (see FIG. 6) may also beformed by the use of porous intermediate layers. For example, pluralityof layers 10 may be formed over substrate 12, layers 10 including gradedlayer 14, relaxed layer 16, and strained layer 18 (see FIG. 1A). Priorto the formation of strained layer 18, a plurality of pores 1614 may beformed in a top portion of relaxed layer 16, thereby defining a porouslayer 1616 in a top portion 1617 of relaxed layer 16. Pores 1614 may beformed by the methods described above with reference to the formation ofpores 1514 in FIG. 15B. Porous layer 1616 may have a thickness T₁₆ of,e.g., 1-5 μm. Strained layer 18 may then be formed directly over porouslayer 1616. Pores 1614 define cleave plane 20 in porous layer 1616.

Referring to FIG. 16B, in an alternative embodiment, after the formationof porous layer 1616 in a portion of relaxed layer 16, a second relaxedlayer 1620 may be formed over relaxed layer 16 including porous layer1616. Second relaxed layer 1620 may include the same material from whichrelaxed layer 16 is formed, e.g., uniform Si_(1-x)Ge_(x) having a Gecontent of, for example, 10-80% (i.e., x=0.1-0.8) and having a thicknessT₁₇ of, e.g., 5-100 nm. In some embodiments, Si_(1-x)Ge_(x) may includeSi_(0.70)Ge_(0.30) and T₁₇ may be approximately 50 nm. Second relaxedlayer 1620 may be fully relaxed, as determined by triple axis X-raydiffraction, and may have a threading dislocation density of <1×10⁶/cm²,as determined by etch pit density (EPD) analysis. Strained layer 18 maybe formed over second relaxed layer 1620. Pores 1614 define cleave plane20 in porous layer 1616.

Referring to FIG. 16C, substrate 12 with layers 10 is bonded to handlewafer 50, including semiconductor substrate 54 and dielectric layer 52,as described with reference to FIG. 3.

Referring to FIG. 16D as well as to FIG. 16C, a split is induced atcleave plane 20 by, for example, cleaving porous layer 1616 by a wateror an air jet. The split results in the formation of two separate wafers1670, 1672. One of these wafers (1670) has top portion 1617 of relaxedlayer 16 (see FIG. 16A) disposed over strained layer 18. Strained layer18 is in contact with dielectric layer 52 on semiconductor substrate 54.The other of these wafers (1672) includes the substrate 12, graded layer14, and a bottom portion 1674 of relaxed layer 16.

Referring to FIG. 6 as well as to FIG. 16D, top portion 1617 of relaxedlayer 16 is removed in any one of the methods described for the removalof relaxed layer portion 80 with reference to FIGS. 4 and 5. Removal oftop portion 1617 of relaxed layer 16 results in the formation of SSOIsubstrate 100.

The bonding of strained silicon layer 18 to dielectric layer 52 has beenexperimentally demonstrated. For example, strained layer 18 having athickness of 54 nanometers (nm) along with ˜350 nm of Si_(0.70)Ge_(0.30)have been transferred by hydrogen exfoliation to Si handle wafer 50having dielectric layer 52 formed from thermal SiO₂ with a thickness ofapproximately 100 nm. The implant conditions were a dose of 4×10¹⁶ions/cm³ of H₂ ⁺ at 75 keV. The anneal procedure was 1 hour at 550° C.to split the SiGe layer, followed by a 1 hour, 800° C. strengtheninganneal. The integrity of strained Si layer 18 and good bonding todielectric layer 52 after layer transfer and anneal were confirmed withcross-sectional transmission electron microscopy (XTEM). An SSOIstructure 100 was characterized by XTEM and analyzed via Ramanspectroscopy to determine the strain level of the transferred strainedSi layer 18. An XTEM image of the transferred intermediate SiGe/strainedSi/Si0 ₂ structure showed transfer of the 54 nm strained Si layer 18 and˜350 nm of the Si_(0.70)Ge_(0.30) relaxed layer 16. Strained Si layer 18had a good integrity and bonded well to SiO₂ 54 layer after theannealing process.

XTEM micrographs confirmed the complete removal of relaxed SiGe layer 16after oxidation and HF etching. The final structure includes strained Silayer 18 having a thickness of 49 nm on dielectric layer 52 includingSiO₂ and having a thickness of 100 nm.

Raman spectroscopy data enabled a comparison of the bonded and cleavedstructure before and after SiGe layer 16 removal. Based on peakpositions the composition of the relaxed SiGe layer and strain in the Silayer may be calculated. See, for example, J. C. Tsang, et al., J. Appl.Phys. 75 (12) p. 8098 (1994), incorporated herein by reference. Thefabricated SSOI structure 100 had a clear strained Si peak visible at˜511/cm. Thus, the SSOI structure 100 maintained greater than 1% tensilestrain in the absence of the relaxed SiGe layer 16. In addition, theabsence of Ge—Ge, Si—Ge, and Si—Si relaxed SiGe Raman peaks in the SSOIstructure confirmed the complete removal of SiGe layer 16.

In addition, the thermal stability of the strained Si layer wasevaluated after a 3 minute 1000° C. rapid thermal anneal (RTA) tosimulate an aggregate thermal budget of a CMOS process. A Ramanspectroscopy comparision was made of SSOI structure 100 as processed andafter the RTA step. A scan of the as-bonded and cleaved sample prior toSiGe layer removal was used for comparision. Throughout the SSOIstructure 100 fabrication process and subsequent anneal, the strained Sipeak was visible and the peak position did not shift. Thus, the strainin SSOI structure 100 was stable and was not diminished by thermalprocessing. Furthermore, bubbles or flaking of the strained Si surface18 were not observed by Nomarski optical microscopy after the RTA,indicating good mechanical stability of SSOI structure 100.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

1. A method comprising: providing a substrate comprising a strainedsemiconductor layer thereover; forming a gate stack comprising a gatedielectric and a gate electrode over the strained semiconductor layer,the gate electrode being over the gate dielectric; and forming a raisedsemiconductor layer over the strained semiconductor layer adjacent tothe gate stack.
 2. The method of claim 1, wherein the gate dielectric isselected from the group consisting essentially of a silicon oxide, agermanium oxide, a germanium oxynitride, a hafnium oxide, a hafniumsilicate, and a combination thereof.
 3. The method of claim 1, whereinthe strained semiconductor layer has a thickness uniformity of betterthan ±5% and a surface roughness of less than 20 Å.
 4. The method ofclaim 1, wherein strained semiconductor layer has a misfit dislocationdensity of less than 10⁵ cm/cm², a threading dislocation density ofbetween 10¹ and 10⁷ dislocations/cm², a surface roughness between 0.1and 1 nm RMS, or a combination thereof.
 5. The method of claim 1 furthercomprising doping the raised semiconductor layer.
 6. The method of claim1 further comprising reacting the raised semiconductor layer with ametal.
 7. The method of claim 1 further comprising relaxing a firstportion of the strained semiconductor layer, wherein a second portion ofthe strained semiconductor layer remains strained.
 8. A methodcomprising: providing a substrate comprising a strained semiconductorlayer, the strained semiconductor layer having a misfit dislocationdensity less than 10⁵ cm/cm² and a threading dislocation density lessthan 10⁷ dislocations/cm²; forming a gate stack comprising a gatedielectric and a gate electrode, the gate dielectric being over thestrained semiconductor layer, the gate electrode being over the gatedielectric; and forming an elevated source/drain region over thestrained semiconductor layer adjacent to the gate stack.
 9. The methodof claim 8, wherein the strained semiconductor layer has a thicknessuniformity of better than ±5% and a surface roughness of less than 20 Å.10. The method of claim 8, wherein strained semiconductor layer has asurface roughness between 0.1 and 1 nm RMS.
 11. The method of claim 8further comprising doping the elevated source/drain region.
 12. Themethod of claim 8 further comprising relaxing a first portion of thestrained semiconductor layer, wherein a second portion of the strainedsemiconductor layer remains strained.
 13. A method comprising: forming arelaxed semiconductor layer over a first substrate; forming a firststrained semiconductor layer over the relaxed semiconductor layer; afterforming the first strained semiconductor layer, forming a cleave planein the relaxed semiconductor layer; bonding the first strainedsemiconductor layer to a second substrate; and inducing a split at thecleave plane in the relaxed semiconductor layer, wherein the firststrained semiconductor layer remains bonded to the second substrate. 14.The method of claim 13 further comprising forming a compositionallygraded layer over the first substrate, the strained semiconductor layerbeing formed over the compositionally graded layer.
 15. The method ofclaim 13 further comprising forming a second strained semiconductorlayer over the relaxed semiconductor layer, the first strainedsemiconductor layer being formed over the second strained semiconductorlayer, the first strained semiconductor layer comprising a differentstrain than the second strained semiconductor layer.
 16. The method ofclaim 13, wherein the forming the cleave plane comprises implanting ionsinto the relaxed semiconductor layer.
 17. The method of claim 16 furthercomprising forming a thin layer over the relaxed semiconductor layer,the implanting the ions being through the thin layer.
 18. The method ofclaim 13 further comprising forming a dielectric layer over the firststrained semiconductor layer, the second substrate, or a combinationthereof, the bonding using the dielectric layer.
 19. The method of claim13, wherein the strained semiconductor layer has an initial misfitdislocation density of less than 10⁵ cm/cm².
 20. The method of claim 13,wherein the strained semiconductor layer has a surface particle densityof less than 0.3 particles/cm².